Use a more distinctive style for the footways.

Since footways and sidewalks are the main feature of this map they need
to stand out from everything else. Solid black lines just looked like
normal road outlines, and the streets without sidewalks just looked
broken.

Now we draw green dotted lines in all of the walkways to make it more
obvious that this is an important element in its own right rather than
just the border of another less-important element.